1. Field of Invention
The present invention relates to power converters, and more particularly, to start-up circuitry and method for a power converter.
2. Description of Related Art
Power converters are essential for many modern electronic devices. Among other capabilities, a power converter can adjust voltage level downward (buck converter and its derivatives) or adjust voltage level upward (boost converter and its derivatives). A power converter may also convert from alternating current (AC) power to direct current (DC) power, or vice versa. A power converter may also function to provide an output at a regulated level (e.g., 5.0V). Power converters are typically implemented using one or more switching devices, such as transistors, which are turned on and off to deliver power to the output of the converter. Control circuitry is provided to regulate the turning on and off of the switching devices, and thus, these converters are known as “switching regulators” or “switching converters.” Such a power converter may be incorporated into or used to implement a power supply—i.e., a switching mode power supply (SMPS). The power converters may also include one or more capacitors or inductors for alternately storing and outputting energy.
Some power converters may employ a startup circuit in order to begin operation after power on. According to previous designs, one kind of startup circuit provides bias power for a controller by charging up a bias capacitor through a high value (“bleeder”) resistor coupled directly between a line voltage and the bias capacitor. The bias capacitor is charged to a voltage level above the under voltage lock out (UVLO) threshold of the controller so that the power converter can begin switching of the switching devices.
Such previous designs are problematic. Even after the high value resistor has served its function during startup, the resistor continuous to cause power loss in the power converter. Thus, during standby or normal operation, the bleeder resistor decreases efficiency. Furthermore, because of safety regulations, the bleeder resistor is typically implemented using two or three discrete resistors. Each such resistor is another part that must be included in implementation for the power converter, thereby increasing the bill of materials (BOM) part count.